1. Field of the Invention
The present invention relates to a radio apparatus for estimating the direction from which a received wave arrives at an antenna and forming a radio transmission channel against a transmitting end of the received wave through the antenna.
2. Description of the Related Art
In recent years, many electronic units are provided with a digital signal processor that performs digital signal processes for many signals in the base band region at high speed, in real time, and at low cost have been outspread.
Thus, for example a radio base station of a mobile communication system that corresponds to the CDMA system actively uses such digital signal processes for not only keeping the directivity of each of a plurality of channels that are formed in parallel by an array antenna composed of a plurality of elements, but also removing interference caused by the directivity and improving the transmission quality of digital signals.
FIG. 8 is a block diagram showing an example of the structure of a radio base station's apparatus that has an array antenna. In the drawing, feeder ends of a plurality of N elements 90E-1 to 90E-N disposed at constant intervals on a virtual straight line are connected to their first input terminals of splitters 91-1 to 91-N, respectively. Output terminals of the splitters 91-1 to 91-N are connected to corresponding input terminals of an arriving angle estimating portion 93 and a beam forming portion 94-R through receiving portions 92-1 to 92-N, respectively. An output terminal of the beam forming portion 94-R is connected to an input terminal of a demodulating portion (not shown). An output terminal of the arriving angle estimating portion 93 is connected to a control input terminal of the beam forming portion 94-R and a control input terminal of a beam forming portion 94-T paired with the beam forming portion 94-R. An input terminal of the beam forming portion 94-T is connected to an output terminal of a modulating portion (not shown). A plurality of N output terminals of the beam forming portion 94-T are connected to first input terminals of signal combing portions 95-1 to 95-N. A single or a plurality of base band signals that will be described later are supplied in parallel to second to p-th (where p≧2) input terminals of each of the signal combining portions 95-1 to 95-N. Output terminals of the signal combining portions 95-1 to 95-N are connected to the other (second) input terminals of the splitters 91-1 to 91-N through transmitting portions 96-1 to 96-N, respectively.
Suffixes of the plurality of elements 90E-1 to 90E-N, splitters 91-1 to 91-N, and receiving portions 92-1 to 92-N are used in common throughout the following description. A combination of an element, a splitter, and a receiving portion that are cascade connected is referred to as a branch.
In the following description, a mutual arrangement of these branches is represented by a physical arrangement of the elements 90E-1 to 90E-N disposed on the foregoing virtual straight line.
In the apparatus of the radio base station, the receiving portions 92-1 to 92-N heterodyne detect (or homodyne detect) received waves that have arrived in parallel at the elements 90E-1 to 90E-N and then input through the splitters 91-1 to 91-N, respectively. As a result, the receiving portions 92-1 to 92-N generate N base band signals S1 to SN, respectively.
In this example, for simplicity, it is assumed that the amplitudes of the base band signals S1 to SN are a common value (“1” normalized by its nominal value). In addition, it is also assumed that the wavelengths of the received waves are a known value λ.
The arriving angle estimating portion 93 performs the following arithmetic operations expressed by the following formulas (1) to (4) to obtain the average value Θ of phase differences of the base band signals S1 to SN obtained through the branches adjacently disposed at constant intervals.
                              S          k                =                                            ⅇ                                                j                  θ                                ⁢                k                                      ⁡                          (                              k                =                                  1                  ⁢                                                                          ⁢                  to                  ⁢                                                                          ⁢                  N                                            )                                                                                          (        1        )                                          ϕ                      k            ,                          k              +              1                                      =                  Arg          ⁡                      (                                          S                                  k                  +                  1                                            ·                              S                k                *                                      )                                              (        2        )                                          θ                      k            ,                          k              +              1                                      =                              sin                          -              1                                ⁡                      (                                                            ϕ                                      k                    ,                                          k                      +                      1                                                                      ·                                  λ                  /                  2                                            ⁢              π              ⁢                                                          ⁢              d                        )                                              (        3        )                                Θ        =                              {                          1              /                              (                                  N                  -                  1                                )                                      }                    ·                                    ∑                              k                =                1                                            N                -                1                                      ⁢                          θ                              k                ,                                  k                  +                  1                                                                                        (        4        )            
In addition, the arriving angle estimating portion 93 obtains an arriving angle ΘA of the received waves as an azimuth angle that is uniquely defined by the average value Θ and the arrangement of the elements 90E-1 to 90E-N (including an azimuth represented by the foregoing virtual straight line).
The beam forming portion 94-R supplies to the demodulating portion a signal obtained as the sum of products of weights ejψ1 to ejψN that represent phase shift amounts ψ1 to ψN against a main lobe of the array antenna 90 in the direction of the arriving angle ΘA and the base band signals S1 to Sn (the resultant signal is hereinafter referred to as base band signal R) so as to form a beam in the base band region.
On the other hand, the beam forming portion 94-T performs the reverse process of the process that the beam forming portion 94-R performs for the base band signal T supplied from the modulating portion. As a result, the beam forming portion 94-T generates N base band signals.
The signal combining portions 95-1 to 95-N combine these N base band signals and sets each of which is composed of a single or a plurality of base band signals, that correspond to the N branches, and that are modulated with transmission information to be transmitted to individual terminals (users) other than the transmitting end of the received wave is transmitted.
The transmitting portions 96-1 to 96-N convert N transmission base band signals into desired radio frequency signals and transmit them in parallel through the splitters 91-1 to 91-N and the elements 90E-1 to 90E-N, respectively.
In other words, since a full duplex radio transmission channel is formed against the transmitting end of the received wave that arrives at the array antenna 90 by the formed beam with a desired directivity, the transmission quality is kept high.
Patent Document 1
Japanese Unexamined Patent Application Publication No. 2002-107439
Patent Document 2
Japanese Unexamined Patent Application Publication No. HEI 10-170621
Patent Document 3
Japanese Unexamined Patent Application Publication No. HEI 6-273504
Patent Document 4
Japanese Unexamined Patent Application Publication No. HEI 8-114662
However, in the foregoing related art references, the accuracy of the arriving angle obtained by the arriving angle estimating portion 93 is sufficiently kept high when the deviations of the phase shift amounts and gains of the receiving portions 92-1 to 92-N are small.
However, the receiving portions 92-1 to 92-N have circuits and devices such as a low noise amplifier (LNA) and a frequency converter whose natural characteristics are nonlinear and involve deviations and whose phase shift amounts and gains may be subject to largely change corresponding to temperature, other environmental conditions, and aged deterioration of the apparatus.
In other words, the arriving angle obtained by the arriving angle estimating portion 93 involves an error and a change. The error and change may cause the transmission quality and the service quality to deteriorate.
These problems would be solved if the receiving portions were designed, produced, set, and kept so that the deviations of the characteristics are very small. Such countermeasures have not been taken because of high cost.
As a related art reference for solving the foregoing problems, there is a patent application filed by the applicant of the present patent application as Japanese Patent Application No. 2001-533594, titled Deviation Compensating Apparatus (translated title).
However, the deviation compensating apparatus needs to have as feed forward circuits dedicated branching circuits, combining circuits, receivers, and so forth. Thus, as the number of elements of an array antenna becomes larger, there is a possibility of which the scale of the hardware increases.